The long-term objectives of this proposal are 1) to improve therapy for non-Hodgkin's lymphoma using combinations of radioimmunotherapy and chemotherapy and 2) to discern the intracellular mechanisms responsible for the synergistic cytotoxicity mediated by these two therapeutic modalities. Preliminary data demonstrate marked synergism in vitro between Iodine-131-radiolabeled anti-CD20 antibody and nucleoside analogs; moderate synergism with topoisomerase inhibitors; and non synergism with cisplatin or 4-hydroxycyclophosphamide. The specific aims of this proposal are four-fold. First, we will test the hypothesis that nucleoside analogs are a more potent class of radiosensitizing drugs for use with I-131-anti-CD20 antibodies than other drug classes using human-derived lymphoma cell lines in vitro. Conclusions will be confirmed in three different in vitro cytotoxic assays, including a clonogenic outgrowth assay, and data will be analyzed by a rigorous isobolographic methodology. Second, we will investigate the mechanisms involved in the potent synergism observed with the nucleoside analogs by assaying well-established effects of the analogs, including the induction of apoptosis, the enhancement of radiation- induced DNA damage, and the incorporation of nucleoside analog into repair DNA. Third, we will investigate the role of Fs-dependent apoptosis in the synergism between anti-CD20 radioimmunotherapy and nucleoside analogs. Fas-dependent and Fas-independent signaling will be studied in experiments employing specific anti-Fas receptor and anti-Fas ligand blocking antibodies, soluble protein inhibitors of the caspase proteins, and immunoblotting of activated forms of caspase 3 (CPP32), caspase 8 (FLICE), and poly-(ADP-ribose) polymerase. Fourth, the most promising combinations of radiolabeled anti-CD20 antibody and chemotherapy will be tested for efficacy in two mouse tumor xenograft models, including a subcutaneous tumor model in nude mice and a disseminated tumor model in SCID mice. The proposed studies will be performed in the laboratory of Dr. Oliver Press at the University of Washington. The large patient base and intense focus on the treatment of lymphomas and other hematologic malignancies at both the University of Washington and the Fred Hutchinson Cancer Research Center and the expertise of the radioimmunotherapy group at these institutions will facilitate rapid translation of promising combinations of radioimmunotherapy and chemotherapy into clinical trials.